1. Field of the Invention
The present invention is in the field of manufacturing luminescent phosphors for cathode ray tubes wherein the raw materials are combined and sintered in combination with a flux containing barium fluoride, barium chloride, or both. The sintering is conducted under sealed conditions, and residual flux is then removed by leaching.
2. Description of the Prior Art
A projection cathode ray tube is excited at a higher level than a conventional cathode ray tube, so that the temperature of the screen is accordingly higher. As a result, the phosphor may become dark, causing temperature quenching and current brightness saturation. In order to prevent this, it has been proposed to eliminate the temperature rise of the phosphor on a projection cathode ray tube by cooling with air by means of a fan or by means of liquid cooling, or to use a phosphor which has excellent current brightness saturation characteristics. However, the results obtained by these measures are not completely satisfactory. Thermal quenching of green is very pronounced, and the white balance is thereby disturbed. When liquid cooling is used, the temperature of the screen of the projection cathode ray tube is raised to as high as 80.degree. C. which results in a reduction of about 1/4 in the initial brightness of the currently used green phosphor (Gd.sub.2 O.sub.2 S:Tb). Even if an attempt is made to excite the phosphor by an increased current density, the currently used phosphor does not have satisfactory current brightness saturation characteristics.
The phosphor composition Y.sub.3 Al.sub.5 O.sub.12 :Tb is produced by the substitution of Tb for Y in a single phase system of Y.sub.3 Al.sub.5 O.sub.12. In the preparation of Y.sub.3 Al.sub.5 O.sub.12, the compounds YAlO.sub.3 and Y.sub.4 Al.sub.2 O.sub.9 tend to be produced simultaneously, and the preparation of a single phase of Y.sub.3 Al.sub.5 O.sub.12 is difficult even if the elements are mixed in stoichiometric amounts.
A report by Naka, Takenaka et al, in Kogyokagaku, Vol. 69, No. 6 (1966) states that the single phase of Y.sub.3 Al.sub.5 O.sub.12 can be prepared by sintering the raw materials at 1490.degree. C. for five hours, pulverizing, mixing, and forming the mixture and then sintering the mixture again at 1490.degree. C. for 24 hours. J. S. Abell et el, reported in J. Mater. Sci., Vol. 9 (1974) that a single phase of Y.sub.3 Al.sub.5 O.sub.12 can be prepared by sintering the raw materials at 1500.degree. C., pulverizing and mixing them, and then sintering the resulting mixture at 1600.degree. C. for two hours. The Tb for substituting a portion of the Y of the single phase is commercially available in the form of Tb.sub.4 O.sub.7, and is not stable in the form of Tb.sup.3+. Therefore, Tb.sup.4+ must be reduced in making the substitution, which further complicates the preparation of the single phase material Y.sub.3 Al.sub.5 O.sub.12 :Tb.